It has been known for a long time to apply electrically conductive and infrared-reflecting tin oxide layers on substrates, particularly glass. In many instances, the preparation of such electrically conductive and infrared reflecting tin-oxide layers is applied to glass by pyrolytic decomposition of liquid preparations which contain essentially one or several tin-based compounds and one or several suitable fluoride-containing doping compounds.
The electrical conductivity is produced by defective areas or regions in the respective tin-oxide layers; such defective areas largely being formed by the added doping agent. In this way, slightly below the conduction band terms of defective areas (or centers of disturbances) or donor terms, respectively, are produced from which, if necessary, with a slight expenditure of energy, electrons can be brought into the conduction band. This is, however, not the case with pure, undoped tin oxide due to the forbidden zone between valence band and conduction band.
Regarding the electrical conductivity or anti-static adjustment of different solid and liquid technical products, such as for example plastics, lacquers, paints, papers, toners and textiles, electrically conductive pigments are necessary to accomplish this task. In addition to metal powders and graphite which necessarily cause a dark colorization of such technical products, powder-shaped semiconductors are often used. In so doing, it is desirable to be able to use semiconductor pigments of high electrical conductivity or low specific resistance which range in color from as white as possible to light-colored and having a small grain size.
In EP-A-0235 968, a transparent, aqueous solution containing a tin compound is disclosed. This solution is obtained by reaction of tin carboxylate, particularly tin oxalate, with hydrogen peroxide in the ratio 1:1.5 or more in an aqueous medium. In this procedure, a doping agent may be contained in the reaction system in an amount of 0.01 to 0.35 mole per mole of tin caboxylate. This transparent aqueous solution is calcined above 400.degree. C.
However, this procedure is not satisfactory in every respect. The tin-II-oxalate which is preferably used as a starting product in the above procedure must first be produced from metallic tin or a tin-II-compound. Tin-II-oxalate, however, is soluble in water with difficulty only, so that the oxidation with H.sub.2 O.sub.2 in an aqueous medium proceeds in a heterogeneous phase.
In order to overcome these disadvantages, it is proposed in the German patent 40 34 353 to prepare the required tin-IV-oxalate solution in the first stage by electrolysis of an aqueous oxalic-acid solution by using anodes of tin or tin-containing alloys.
If tin-IV-oxide or the corresponding tin-IV-oxide hydrate is precipitated from these solutions, for example, by adding alkali hydroxide or ammonium hydroxide, then oxalate-containing waste waters are obtained which must be disposed of in an environmentally-acceptable way. If the above-mentioned suspensions are dehydrated by spray drying, then tin-IV-oxide is obtained in a predominantly spherical form. From an application technical point of view, however, it is desirable to obtain particles of amorphous tin-IV-oxide in a shape which can be varied to suit a particular purpose.